March 1, 2010

The AMO in the CET?

A couple of weeks ago a commenter suggested I “personally check” the “statistical significance” of “temperature trends”. Well, I’m not sure that’s really my role in life, but as it happens I did take a peek at the Central England Temperature (CET) record last week to see if I could see any evidence for an AMO (Atlantic Multi-Decadal Oscillation – see previousposts), which hypothetically alternately masks (e.g. ~1940-70) and reinforces (e.g. ~1970-2000) global warming and may even overshoot, producing some cooling.

My hypothesis is that the AMO drives the Arctic Oscillation (AO) (measured by the NAO or the NAM) which determines the nature of UK winters. Cold winters, influenced by the high pressure over Greenland and Scandinavia occur during the negative phase of the AO when there is low pressure in the Arctic which itself occurs, I propose, when the Arctic is relatively warmer, as we might expect around the time when sea ice cover is at a minimum (2007 in the current cycle).

A testable prediction might therefore be of cyclic severity of UK winters, as measured by the CET.

The graphs on the Met. Office site only cover the period from 1772, yet the record extends back to 1659. I therefore found myself importing the raw data into Excel. Here’s the result of my first plot:

[Btw I just found that, to avoid corrupting its appearance, I had to export the chart from Excel to Word to Powerpoint to a JPG and then import here using WordPress facilities. Damn you Bill Gates!]

First, a couple of comments about the plot:

I’ve included running means calculated for the years either side of a given year (i.e. not trailing). So, the 21 year mean for 1900, for example, is the mean of the temperatures for 1890 to 1910, inclusive.

2010 data is not yet included – the down-leg at the extreme right is 2009 (average temperature 3.53C).

Clearly statistical analysis will be necessary to prove anything, and I’m sure lifetimes have been spent sifting through this data. Nevertheless, it is possible to make a few initial observations:

There’s no clear evidence of a short-term link between volcanic activity and extreme winters in the CET temperature record (maybe I’ll look at annual temperatures and other seasons another time), with the exception of a run of cold winters following the eruption of Laki in Iceland in 1783-4. But there are other similar series of cold winters when there was no volcanic forcing, for example from 1939-42. Krakatoa (1883) caused measurable global cooling, but occurred in the middle of a run of relatively mild winters in the CET. Tambora occurred after the exceptionally cold winter of 1813-14, although there is evidence of another major eruption in 1810. More recently Agung occurred after the famously cold winter of 1962-3 and other eruptions have similarly had little apparent effect on succeeding average winter temperatures in the CET. This supports the hypothesis that the winter CET record is primarily influenced by weather patterns – central England is mild in winter when the Atlantic influence on the weather dominates, and cold when Continental air flows over the Britich Isles. Winter temperatures in the CET may therefore be a valid proxy for the Arctic Oscillation, as I proposed at the outset.

There appears to be an underlying warming trend in the data. During the 20th century the 21 year mean always remained around a degree above the low-point in the 17th century.

There have been two or three previous periods (e.g. ~1680 – 1730s, 1890s – ~1920) of increasingly milder on average winters as well as that leading up to the first decade of the 21st century (the 2000s).

There have been runs of exceptionally mild winters prior to that in the 2000s, though the 2000s were slightly warmer than the antecedents. The 1730s stand out, but there were also runs of mild winters in the 1910s and 1970s. In fact, there are so many runs of mild winters that I feel obliged to point out that – if variation in winter temperature were distributed randomly – we would expect 8 milder than average winters in a row to occur only once in 256 years (strictly 256 sets of 8 years which would need 263 years of data – we have 350), 9 once in half a millennium and 10 once a millennium. Without wishing to commit myself, a closer look at the data may be in order (note that the presentation here slightly underestimates the length of warm and cold sequences, since a 21 year running mean is too short not to be dragged upwards by the series – 8 to 10 mild years – of data we’re looking for).

Long sequences of colder than average winters seem to be rarer than those of mild winters, but maybe this is because cold winters tend to represent a larger deviation from the mean than do warm winters.

There are cases (e.g. 1739-40, 1962-3) of exceptionally cold winters occurring soon after milder periods and of mild winters (e.g. 1685-6, 1795-6) following cold ones. Perhaps the first question to ask is whether the data is random. If it’s not, then we can start to try to work out what sort of oscillations there are in the system.

[…] Why there’s a Year Zero in 1910 is beyond me. I’ll let you know when I find out. Presumably someone has decided that records are unreliable before that point, despite the tens of thousands of hours of effort that have gone into constructing the Central England Temperature (CET) record which goes back to 1659. I can believe that the monthly averages are off by 0.1 or 0.2C, but they’re going to be good enough for the purposes of comparison. Regular readers will be aware that I have imported the CET data into Excel. […]

The 1810 volcanic eruption does seem to have had a major impact on summer CETs – which for the curious are much more critical for creating ice ages than winter temperatures. (Consider how with exceptionally cold winters Siberia and Manchuria were never glaciated because they had vastly less snow and warmer summers than Europe).

The period from 1770 to 1809 is notable for having numerous cold winters and hot summers. Two peak periods from 1772 to 1788 and the decade of the 1800s possessed more frequent extremely hot summers than anything before global warming impacted the climate. Between 1772 and 1808 sixteen of 37 summers were in the top quartile, with the hot summers of 1781 and 1808 particularly outstanding. 1781 was the hottest summer in the series to that point, and aided by an exceptional May and stormy July, 1808 is still the ninth-hottest May to August on record. In contrast, only two summers in that era were in the bottom quartile (and one of those, 1784, had a very hot May and September).

In contrast to the hot summers, the 1800s and 1780s are notable for completely lacking seriously mild winters. No winter from the 1780s, 1800s or 1810s is in the top quartile – though the “bookend” mild winters of this drought, 1795/1796 and 1821/1822, were both quite exceptional. In fact, July 1821 to June 1822 (CET 10.72˚C) stood as the warmest fiscal year in the CET series until 1990.

The eighty to eighty-five years following 1810 showed a clear prevalence of cool summers. Of the fourteen summers with CET below 14˚C, seven occurred between 1810 and 1890 vis-à-vis none between 1730 and 1810. There were three main peaks in the frequency of cool summers – between 1809 and 1823, 1839 and 1854, and 1879 and 1894. The decades from 1839 to 1848 and 1812 to 1821 each contained seven summers in the lowest quartile, as against only six amongst the eighty-three summers from 1726 to 1808. It seems to me that long runs of cool summers (six of seven in the first half of the 1840s) may be naturally more likely than long runs of hot summers though this may be less definitive than for winters.

Only nineteen summers from 1809 to 1895 were in the top quartile, although the summer of 1826 was the hottest until 1976 and is a case of an exceptionally hot summer after a very cool period, as was the summer of 1846.